Land issue

My father,uneducated poor soul,bought a land from a family 15 years ago.The land purchase was signed by all the members of the family in which one boy was a minor.We did not know about that untill now.The boy is blackmailing us recently  to give 1/3rd of the land rate now as his sign  is invalid because he was a minor and that he could file a case and get his share of land(ie 1/3) .Now what should we do.Please kindly provide a suggestion.